Chapter XVII: Boron, Aluminium, and the Analogous Metals of the Third Group

If the elements of small atomic weight which we have hitherto discussed be placed in order, it will be clearly seen that, judging by the formulæ of their higher compounds, one element is wanting between beryllium and carbon. For lithium gives LiX, beryllium forms BeX_{2}, and then comes carbon giving CX_{4}. Evidently to complete the series we must look for an element forming RX_{3}, and having an atomic weight greater than 9 and less than 12. And _boron_ is such a one; its atomic weight is 11, and its compounds are expressed by BX_{3}. Lithium and beryllium are metals; carbon has no metallic properties; boron appears in a free state in several forms which are intermediate between the metals and non-metals. Lithium gives an energetic caustic oxide, beryllium forms a very feeble base; hence one would expect to find that the oxide of boron, B_{2}O_{3}, has still more feeble basic properties and some acid properties, all the more as CO_{2} and N_{2}O_{5}, which follow after B_{2}O_{3} in their composition and in the periodic system, are acid oxides. And, indeed, the only known _oxide of boron_ exhibits a feeble basic character, together with the properties of a feeble acid oxide. This is even seen from the fact that a solution of boron oxide reddens blue litmus and acts on turmeric paper as an alkali, and these reactions may be used for determining the presence of B_{2}O_{3} in solutions. By themselves the alkali borates have an alkaline reaction, which clearly indicates the feeble acid character of boric acid. If they are mixed in solution with hydrochloric acid, boric acid is liberated, and if a piece of turmeric paper be immersed in this solution and then dried, the excess of hydrochloric acid volatilises, while the boric acid remains on the paper and communicates a _brown coloration_ to it, just like alkalis.

Boron trioxide or boric anhydride enters into the composition of many minerals, in the majority of cases in small quantities as an isomorphous admixture, not replacing acids but bases, and most frequently alumina (Al_{2}O_{3}), for as a rule the amount of alumina decreases as that of the boric anhydride increases in them. This substitution is explained by the similarity between the atomic composition of the oxides of aluminium (alumina) and boron. The subdivision of oxides into basic and acid can in no way be sharply defined, and here we meet with the most conclusive proof of the fact, for the oxides of boron and aluminium belong to the number of intermediate oxides, closely approaching the limit separating the basic from the acid oxides. Their type (Chapter XV.) R_{2}O_{3} is intermediate between those of the basic oxides R_{2}O and RO and those of the acid oxides R_{2}O_{5} and RO_{3}. If we turn our attention to the chlorides, we remark that lithium chloride is soluble in water, is not volatile, and is not decomposed by water; the chlorides of beryllium and magnesium are more volatile, and although not entirely, still are decomposed by water; whilst the chlorides of boron and aluminium are still more volatile and are decomposed by water. Thus the position of boron and aluminium in the series of the other elements is clearly defined by their atomic weights, and shows us that we must not expect any new and distinct functions in these elements.

Boron was originally known in the form of sodium borate, Na_{2}B_{4}O_{7},10H_{2}O, or _borax_, or _tincal_, which was exported from Asia, where it is met with in solution in certain lakes of Thibet; it has also been discovered in California and Nevada, U.S.A.[1] Boric acid was afterwards found in sea-water and in certain mineral springs.[2] Its presence may be discovered by means of the green coloration which it communicates to the flame of alcohol, which is capable of dissolving free boric acid.[3] Many of the boron compounds employed in the arts are obtained from the impure boric acid which is extracted in Tuscany from the so-called _suffioni_. In these localities, which present the remains of volcanic action, steam mixed with nitrogen, hydrogen sulphide, small quantities of boric acid, ammonia, and other substances, issue from the earth.[3 bis] The boric acid partially volatilises with the steam, for if a solution of boric acid be boiled, the distillate will always contain a certain amount of this substance.[4]

[1] Borax is either directly obtained from lakes (the American lakes
give about 2,000 tons and the lakes of Thibet about 1,000 tons per
annum), or by heating native calcium borate (_see_ Note 2) with
sodium carbonate (about 4,000 tons per annum), or it is obtained
(up to 2,000 tons) from the Tuscan impure boric acid and sodium
carbonate (carbonic anhydride is evolved). Borax gives
supersaturated solutions with comparative ease (Gernez), from which
it crystallises, both at the ordinary and higher temperatures, in
octahedra, containing Na_{2}B_{4}O_{7},5H_{2}O. Its sp. gr. is
1·81. But if the crystallisation proceeds in open vessels, then at
temperatures below 56°, the ordinary prismatic crystallo-hydrate
B_{4}Na_{2}O_{7},10H_{2}O is obtained. Its sp. gr. is 1·71, it
effloresces in dry air at the ordinary temperature, and at 0° 100
parts of water dissolve about 8 parts of this crystallo-hydrate, at
50° 27 parts, and at 100° 201 parts. Borax fuses when heated, loses
its water and gives an anhydrous salt which at a red heat fuses
into a mobile liquid and solidifies into a transparent amorphous
_glass_ (sp. gr. 2·37), which before hardening acquires the pasty
condition peculiar to common molten glass. Molten borax dissolves
many oxides and on solidifying acquires characteristic tints with
the different oxides; thus oxide of cobalt gives a dark blue glass,
nickel a yellow, chromium a green, manganese an amethyst, uranium a
bright yellow, &c. Owing to its fusibility and property of
dissolving oxides, borax is employed in soldering and brazing
metals. Borax frequently enters into the composition of strass and
fusible glasses.

[2] We may mention the following among the minerals which contain
boron: calcium borate, (CaO)_{3}(B_{2}O_{3})(H_{2}O)_{6}, found and
extracted in Asia Minor, near Brusa; _boracite_ (stassfurtite),
(MgO)_{6}(B_{2}O_{3})_{8},MgCl_{2}, at Stassfurt, in the regular
system, large crystals and amorphous masses (specific gravity
2·95), used in the arts; _ereméeffite_ (Damour), AlBO_{3} or
Al_{2}O_{3}B_{2}O_{3}, found in the Adulchalonsk mountains in
colourless, transparent prisms (specific gravity 3·28) resembling
apatite; _datholite_, (CaO)_{2}(SiO_{2})_{2}B_{2}O_{3},H_{2}O; and
ulksite, or the boron-sodium carbonate from which a large quantity
of borax is now extracted in America (Note 1). As much as 10 p.c.
of boric anhydride sometimes enters into the composition of
tourmalin and axinite.

[3] This green coloration is best seen by taking an alcoholic solution
of volatile ethyl borate, which is easily obtained by the action of
boron chloride on alcohol.

[3 bis] P. Chigeffsky showed in 1884 (at Geneva) that in the
evaporation of saline solutions many salts are carried off by the
vapour--for instance, if a solution of potash containing about
17-20 grams of K_{2}CO_{3} per litre be boiled, about 5 milligrams
of salt are carried off for every litre of water evaporated. With
Li_{2}CO_{3} the amount of salt carried over is infinitesimal, and
with Na_{2}CO_{3} it is half that given by K_{2}CO_{3}. The
volatilisation of B_{2}O_{3} under these circumstances is
incomparably greater--for instance, when a solution containing 14
grams of B_{2}O_{3} per litre is boiled, every litre of water
evaporated carries over about 350 milligrams of B_{2}O_{3}. When
Chigeffsky passed steam through a tube containing B_{2}O_{3} at
400°, it carried over so much of this substance that the flame of a
Bunsen's burner into which the steam was led gave a distinct green
coloration; but when, instead of steam, air was passed through the
tube there was no coloration whatever. By placing a tube with a
cold surface in steam containing B_{2}O_{3}, Chigeffsky obtained a
crystalline deposit of the hydrate B(OH)_{3} on the surface of the
tube. Besides this, he found that the amount of B_{2}O_{3} carried
over by steam increases with the temperature, and that crystals of
B(OH)_{3} placed in an atmosphere of steam (although perfectly
still) volatilise, which shows that this is not a matter of
mechanical transfer, but is based on the capacity of B_{2}O_{3} and
B(OH)_{3} to pass into a state of vapour in an atmosphere of steam.

[4] How it is that these vapours containing boric acid are formed in
the interior of the earth is at present unknown. Dumas supposes
that it depends on the presence of _boron sulphide_, B_{2}S_{3}
(others think boron nitride), at a certain depth in the earth. This
substance may be artificially prepared by heating a mixture of
boric acid and charcoal in a stream of carbon bisulphide vapour,
and by the direct combination of boron and the vapour of sulphur at
a white heat. The almost non-crystalline compound B_{2}S_{3}, sp.
gr. 1·55, thus obtained is somewhat volatile, has an unpleasant
smell, and is very easily decomposed by water, forming boric acid
and hydrogen sulphide, B_{2}S_{3} + 3H_{2}O = B_{2}O_{3} + 3H_{2}S.
It is supposed that a bed of boron sulphide lying at a certain
depth below the surface of the earth comes into contact with sea
water which has percolated through the upper strata, becomes very
hot, and gives steam, hydrogen sulphide, and boric acid. This also
explains the presence of ammonia in the vapours, because the sea
water certainly passes through crevices containing a certain amount
of animal matter, which is decomposed by the action of heat and
evolves ammonia. There are several other hypotheses for explaining
the presence of the vapours of boric acid, but owing to the want of
other known localities the comparison of these hypotheses is at
present hardly possible. The amount of boric anhydride in the
vapours which escape from the Tuscan fumerolles and suffioni is
very inconsiderable, less than one-tenth per cent., and therefore
the direct extraction of the acid would be very uneconomical, hence
the heat contained in the discharged vapours is made use of for
evaporating the water. This is done in the following manner.
Reservoirs are constructed over the crevices evolving the vapours,
and the water of some neighbouring spring is passed into them. The
vapours are caused to pass through these reservoirs, and in so
doing they give up all their boric acid to the water and heat it,
so that after about twenty-four hours it even boils; still this
water only forms a very weak solution of boric acid. This solution
is then passed into lower basins and again saturated by the vapours
discharged from the earth, by which means a certain amount of the
water is evaporated and a fresh quantity of boric acid absorbed;
the same process is repeated in another reservoir, and so on until
the water has collected a somewhat considerable amount of boric
acid. The solution is drawn from the last reservoir A into settling
vessels B D, and then into a series of vessels _a_, _b_, _c_. In
these vessels, which are made of lead, the solution is also
evaporated by the vapours escaping from the earth, and attains a
density of 10° to 11° Baumé. It is allowed to settle in the vessel
C, in which it cools and crystallises, yielding (not quite pure)
crystalline boric acid. At temperatures above 100°, for instance,
with superheated steam, boric acid volatilises with steam very
easily.

If boric acid be introduced into an excess of a strong hot solution of sodium hydroxide, then, on slowly cooling, the salt NaBO_{2},4H_{2}O crystallises out. This salt contains an equivalent of Na_{2}O to one equivalent B_{2}O_{3}. It might be termed a neutral salt did it not possess strongly alkaline reactions and easily split up into the alkali and the more stable borax or biborate of sodium mentioned above, which contains 2B_{2}O_{3} to Na_{2}O.[5] This salt is prepared by the action of boric acid on a solution of sodium carbonate. Borax may be perfectly purified by crystallisation. If a saturated and hot solution of borax be mixed with strong hydrochloric acid, common salt and a normal crystalline hydrate of _boric acid_ are formed. The composition of this hydrate is B(HO)_{3}, according to the form BX_{3}--that is, of the composition B_{2}O_{3},3H_{2}O. This is the easiest method of obtaining pure boric acid. The water is easily expelled from this hydrate; it loses half at 100° and the remainder on further heating, and the remaining B_{2}O_{3} or boric anhydride fuses at 580° (according to Carnelley), forming at first a ductile (easily drawn out into threads), tenacious mass and then a colourless liquid solidifying to a transparent glass, which absorbs moisture from the atmosphere and then becomes cloudy.[6] Only the alkaline salts of boric acid are soluble in water, but all borates are soluble in acids, owing to their easy decomposability and the solubility of boric acid itself. Although boric anhydride, B_{2}O_{3}, absorbs 3H_{2}O from damp air, still in the presence of water it always[7] combines with a less quantity of bases (borax only contains 1/6). However, fused boric anhydride forms a crystalline compound with magnesium of the same type as the hydrate (MgO)_{3}B_{2}O_{3} (Ebelmann), and even with sodium it forms (Na_{2}O)_{3}B_{2}O_{3} or Na_{3}BO_{3} (Benedict). As a rule, the salts of boric acid contain less base, although they are all able to form saline compounds with bases when fused. Generally, vitreous fluxes are formed by this means,[8] which when fused recall ordinary aqueous solutions in many respects. Some of them crystallise on solidifying, and then they have, like salts, a definite composition. The property of boric anhydride of forming higher grades of combination with basic oxides when fused explains the power of fused borax to dissolve metallic oxides, and the experiments of Ebelmann on the preparation of artificial crystals of the precious stones by means of boric anhydride. Boric anhydride is, although with difficulty, volatile at a high temperature, and therefore if it dissolves an oxide, it may be partially driven off from such a solution by prolonged and powerful ignition; in which case the oxides previously in solution separate out in a crystalline form, and frequently in the same forms as those in which they occur in nature--for example, crystals of alumina, which by itself fuses with difficulty, have been obtained in this manner. It dissolves in molten boric anhydride, and separates out in natural rhombohedric crystals. In this way Ebelmann also obtained _spinel_--that is, a compound of magnesium and aluminium oxides which occurs in nature.[9]

[5] Metals, like Na, K, Li, give salts of the type of borax, MBO_{2} or
MH_{2}BO_{3}. A solution of borax, Na_{2}B_{4}O_{7}, has an
alkaline reaction, decomposes ammonia salts with the liberation of
ammonia (Bolley), absorbs carbonic anhydride like an alkali,
dissolves iodine like an alkali (Georgiewics), and seems to be
decomposed by water. Thus Rose showed that strong solutions of
borax give a precipitate of silver borate with silver nitrate,
whilst dilute solutions precipitate silver oxide, like an alkali.
Georgiewics even supposes (1888) boric anhydride to be entirely
void of acid properties; for all acids, on acting on a mixture of
solutions of potassium iodide and iodate, evolve iodine, but boric
acid does not do this. With dilute solutions of sodium hydroxide
Berthelot obtained a development of heat equal to 11-1/2 thousand
calories per equivalent of alkali (40 grams sodium hydroxide) when
the ratio Na_{2}O : 2B_{2}O_{3} (as in borax) was taken, and only 4
thousand calories when the ratio was Na_{2}O : B_{2}O_{3}, whence
he concludes that water powerfully decomposes those sodium borates
in which there is more alkali than in borax. Laurent (1849)
obtained a sodium compound, Na_{2}O,4B_{2}O_{3},10H_{2}O,
containing twice as much boric anhydride as borax, by boiling a
mixture of borax with an equivalent quantity of sal-ammoniac until
the evolution of ammonia entirely ceased.

Hence it is evident that feeble acids are as prone to, and as
easily, form acid salts (that is, salts containing much acid oxide)
as feeble bases are to give basic salts. These relations become
still clearer on an acquaintance with such feeble acids as silicic,
molybdic, &c. This variety of the proportions in which bases are
able to form salts recalls exactly the variety of the proportions
in which water combines with crystallo-hydrates. But the want of
sufficient data in the study of these relations does not yet permit
of their being generalised under any common laws.

With respect to the feeble acid energy of boric anhydride I think
it useful to add the following remarks. Carbonic anhydride is
absorbed by a solution of borax, and displaces boric anhydride; but
it is also displaced by it, not only on fusion, but also on
solution, as the preparation of borax itself shows. Sulphuric
anhydride is absorbed by boric acid, forming a compound
B(HSO_{4})_{3}, where HSO_{4} is the radicle of sulphuric acid
(D'Ally). With phosphoric acid, boric acid forms a stable compound,
BPO_{4}, or B_{2}O_{3}P_{2}O_{5}, undecomposable by water, as
Gustavson and others have shown. With respect to tartaric acid,
boric anhydride is able to play the same part as antimonious oxide.
Mannitol, glycerol, and similar polyhydric alcohols also seem able
to form particularly characteristic compounds with boric anhydride.
All these aspects of the subject require still further explanation
by a method of fresh and detailed research.

[6] Ditte determined the sp. gr.:--

0° 12° 80°
B_{2}O_{3} 1·8766 1·8470 1·6988
B(OH)_{3} 1·5463 1·5172 1·3828
Solubility 1·95 2·92 16·82

The last line gives the solubility, in grams, of boric acid,
B(OH)_{3}, per 100 c.c. of water, also according to the
determinations of Ditte.

[7] It is evident that, in the presence of basic oxides, water competes
with them, which fact in all probability determines both the amount
of water in the salts of boric acid as well as their decomposition
by an excess of water. In confirmation of the above-mentioned
competing action between water and bases, I think it useful to
point out that the crystallo-hydrate of borax containing 5H_{2}O
may be represented as B(HO)_{3}, or rather as B_{2}(OH)_{6}, with
the substitution of one atom of hydrogen by sodium, since
Na_{2}B_{4}O_{7},5H_{2}O = 2B_{2}(OH)_{5}(ONa). The composition of
the acid boric salts is very varied, as is seen from the fact that
Reychler (1893) obtained (Cs_{2}O)3B_{2}O_{3}, (Rb_{2}O)2B_{2}O_{3}
(corresponding to borax) and (Li_{2}O)B_{2}O_{3}, and that Le
Chatelier and Ditte obtained, for CaO, MgO, &c., (RO)B_{2}O_{3},
(RO)_{2}3B_{2}O_{3}, (RO)2B_{2}O_{3}, (RO)_{2}B_{2}O_{3}, and even
(RO)_{3}B_{2}O_{3}.

[8] A glass can only be formed by those slightly volatile oxides which
correspond with feeble acids, like silica, phosphoric and boric
anhydrides, &c., which themselves give glassy masses, like quartz,
glacial phosphoric acid, and boric anhydride. They are able, like
aqueous solutions and like metallic alloys, to solidify either in
an amorphous form or to yield (or even be wholly converted into)
definite crystalline compounds. This view illustrates the position
of solutions amongst the other chemical compounds, and allows all
alloys to be regarded from the aspect of the common laws of
chemical reactions. I have therefore frequently recurred to it in
this work, and have since the year 1850 introduced it into various
provinces of chemistry.

[9] If boric acid in its aqueous solutions proves to be exceedingly
feeble, unenergetic, and easily displaced from its salts by other
acids, yet in an anhydrous state, as anhydride, it exhibits the
properties of an energetic acid oxide, and it _displaces_ the
anhydrides of other acids. This of course does not mean that the
acid then acquires new chemical properties, but only depends on the
fact that the anhydrides of the majority of acids are much more
volatile than boric anhydride, and therefore the salts of many
acids--even of sulphuric acid--are decomposed when fused with boric
anhydride.

By itself boric acid is used in the arts in small quantity, chiefly
for the preservation of meat and fish (which must be afterwards
well washed in water) and of milk, and for soaking the wicks of
stearin candles; the latter application is based on the fact that
the wicks, which are made of cotton twist, contain an ash which is
infusible by itself but which fuses when mixed with boric acid.

Free _boron_ was obtained (1809) by Davy, Gay-Lussac, and Thénard when they obtained the metals of the alkalis, for boric anhydride when fused with sodium gives up its oxygen to the sodium, and free boron is liberated as an _amorphous_ powder like charcoal.[10] It is of a brown colour, specific gravity 2·45 (Moissan), and when dry does not alter in the air at the ordinary temperature; but it burns when ignited to 700°, and in so doing combines not only with the oxygen of the air, but also with the nitrogen. However, the combustion is never complete, because the boric anhydride formed on the surface covers the remaining mass of the boron, and so preserves it from the action of the oxygen. Acids, even sulphuric (forming SO_{2}) and phosphoric (forming phosphorus), easily oxidise amorphous boron, especially when heated, converting it into boric acid. Alkalis have the same action on it, only in this case hydrogen is evolved. Boron decomposes steam at a red heat, also with evolution of hydrogen.

[10] _Amorphous boron_ is prepared by mixing 100 parts of powdered
boric anhydride with 50 parts of sodium in small lumps; this
mixture is thrown into a powerfully heated cast-iron crucible,
covered with a layer of ignited salt, and the crucible covered.
Reaction proceeds rapidly; the mass is stirred with an iron rod,
and poured directly into water containing hydrochloric acid. The
action is naturally accompanied by the formation of sodium borate,
which is dissolved, together with the salt, by the water, whilst
the boron settles at the bottom of the vessel as an insoluble
powder. It is washed in water, and dried at the ordinary
temperature. Magnesium, and even charcoal and phosphorus, are also
able to reduce boron from its oxide. Boron, in the form of an
amorphous powder, very easily passes through filter-paper, remains
suspended in water, and colours it brown, so that it appears to be
soluble in water. Sulphur precipitated from solutions shows the
same (colloidal) property. When borax is fused with magnesium
powder, it gives a brown powder of a compound of boron and
magnesium, Mg_{2}B (Winkler, 1890), but when a mixture of 1 part
of magnesium and 3 parts of B_{2}O_{3} is heated to redness
(Moissan, 1892), it forms amorphous boron in the form of a
chestnut-coloured powder, which, after being washed with water,
hydrochloric and hydrofluoric acids, is fused again with
B_{2}O_{3} in an atmosphere of hydrogen in order to prevent the
access of the nitrogen of the air, which is easily absorbed by
incandescent amorphous boron.

Sabatier (1891) considers that a certain amount of gaseous hydride
of boron is evolved in the action of hydrochloric acid upon the
alloys of magnesium and boron, because the gas disengaged burns
with a green flame. Still, the existence of hydride of boron
cannot be regarded as certain.

Under the action of the heat of the electric furnace boron forms
with carbon a _carbide_, BC, as Mühlhäuser and Moissan showed in
1893.

Amorphous boron, like charcoal, dissolves in certain molten metals. The property of fused _aluminium of dissolving boron_ in considerable quantity is very striking; on cooling such a solution, the boron partially combined with the aluminium separates out in a crystalline form, and its properties are then exceedingly remarkable. The crystalline boron may be obtained by heating (to 1,300°) the pulverulent boron with aluminium in a well-closed crucible, the access of air being prevented as far as possible. After cooling, crystals are observed on the surface of the aluminium, and may easily be separated by dissolving the latter in hydrochloric acid, which does not act on the crystals. The specific gravity of the crystals is 2·68; they are partially transparent, but are for the most part coloured dark brown; they contain about 4 p.c. of carbon and up to 7 p.c. of aluminium, so that they cannot be considered as pure boron. Nevertheless, the properties of this _crystalline_ substance, which was obtained by Wöhler and Deville, are very remarkable. It most closely resembles _the diamond in its properties_--in fact, these crystals have the lustre and high refracting power proper to the diamond only, whilst their hardness competes with that of the diamond. Their powder polishes even the diamond, and like the diamond scratches the sapphire and corundum. Crystalline boron is much more stable with respect to chemical reagents than the amorphous variety, and as it resembles the diamond, so amorphous boron, on the other hand, distinctly recalls certain of the properties of charcoal; thus a certain resemblance exists between boron and carbon in a free state, which is further justified by the proximity of their positions in the periodic system.

Among the other compounds of boron, those with nitrogen and the halogens are the most remarkable. As already mentioned above, amorphous boron combines directly with _nitrogen_ at a red heat. If it be heated in a glass tube in a stream of nitric oxide, perfect combustion takes place, 5B + 3NO = B_{2}O_{3} + 3BN. If the residue be treated with nitric acid, the boric anhydride dissolves, whilst the _boron nitride_ remains[11] as an extremely light white powder, which is sometimes partially crystalline and greasy to the touch, like talc. It is infusible and unchanged, even at the melting-point of nickel. In general, it is remarkable for its great stability with respect to chemical reagents. Nitric and hydrochloric acids, as well as alkaline solutions, and hydrogen and chlorine at a red heat, have no action on it. When fused with potash, it evolves ammonia, and when ignited in steam it also yields ammonia: 2BN + 3H_{2}O = B_{2}O_{3} + 2NH_{3}.[12]

[11] At first boron nitride was obtained by heating boric acid with
potassium cyanide or other cyanogen compounds. It may be more
simply prepared by heating anhydrous borax with potassium
ferrocyanide, or by heating borax with ammonium chloride. For this
purpose one part of borax is intimately mixed with two parts of
dry ammonium chloride, and the mixture heated in a platinum
crucible. A porous mass is formed, which after crushing and
treating with water and hydrochloric acid, leaves boron nitride.
_Boron fluoride_, BF, is known, corresponding to BN; this body was
obtained by Besson and Moissan (1891). The action of phosphorus
upon iodide of boron, BI_{3}, forms PBI_{2}, and when heated to
500° in hydrogen it forms BP, which gives PH_{3} with fused KHO.

[12] When fused with potassium carbonate it forms potassium cyanate,
BN + K_{2}CO_{3} = KBO_{2} + KCNO. All this shows that boron
nitride is a nitrile of boric acid, BO(OH) + NH_{3} - 2H_{2}O =
BN. The same is expressed by saying that boron nitride is a
compound of the type of the boron compounds BX_{3}, with the
substitution of X_{3} by nitrogen, as the trivalent radicle of
ammonia, NH_{3}.

No less remarkable is the compound of boron with fluorine--_boron fluoride_, BF_{3}. It is produced in many instances when compounds of boron and of fluorine are brought together.[13] The most convenient method of preparing it is by heating a mixture of calcium fluoride with boric anhydride and sulphuric acid, 3CaF_{2} + B_{2}O_{3} + 3H_{2}SO_{4} = 3CaSO_{4} + 3H_{2}O + 2BF_{3}.[14] It is a colourless liquefiable _gas_ (the liquid boils at -100°), which on coming into contact with damp air forms white fumes, owing to its combining with water. One volume of water dissolves as much as 1,050 volumes of this gas (Bazaroff), forming a liquid which disengages boron fluoride when heated, and distils over unaltered. Boron fluoride chars organic matter, owing to its taking up the water from it, and in this respect it acts like sulphuric acid. The behaviour of boron fluoride with water must be understood as a reversible reaction, since with water it yields hydrofluoric and boric acids, whilst they, acting on one another, re-form boron fluoride and water. A state of equilibrium is set up between these four substances (and between two reversible reactions) which is distinctly dependent on the mass of the water.[14 bis] When boron fluoride is in great excess, the equilibrated system, which is capable of distilling over (sp. gr. of the liquid 1·77), has a composition BF_{3},2H_{2}O (or B_{2}O_{3},H_{2}O,6HF). It has also its corresponding salts.[15] It is a caustic liquid, having the properties of a powerful acid; but it does not act on glass, which shows that there is no free hydrofluoric acid present. Under the action of water this system changes, with the formation of boric acid and hydroborofluoric acid (HBF_{4}) according to the equation 4BF_{3}H_{4}O_{2} = 3HBF_{4} + BH_{3}O_{3} + 5H_{2}O.[16] This hydroborofluoric acid has its corresponding salts--for instance, KBF_{4}. On evaporating the aqueous solution this free acid decomposes, with the evolution of hydrofluoric acid, and a stable system is again obtained: 2HBF_{4} + 5H_{2}O = B_{2}F_{6}H_{10}O_{5} + 2HF. The resultant solution (containing 2BF_{3},5H_{2}O, sp. gr. 1·58), which is identical with that formed by the evaporation of a solution of boric acid with hydrofluoric acid, again only contains a compound of boron fluoride with water. Probably there are various other possible and more or less stable states of equilibrium and definite compounds of boron fluoride, hydrofluoric acid, and water.

[13] Boron fluoride is frequently evolved on heating certain compounds
occurring in nature containing both boron and fluorine. If calcium
fluoride is heated with boric anhydride, calcium borate and boron
fluoride are formed, and the latter, as a gas, is volatilised:
2B_{2}O_{3} + 3CaF_{2} = 2BF_{3} + Ca_{3}B_{2}O_{6}. The calcium
borate, however, retains a certain amount of calcium fluoride.

[14] In order to avoid the formation of silicon fluoride the
decomposition should not be carried on in glass vessels, which
contain silica, but in lead or platinum vessels. Boron fluoride by
itself does not corrode glass, but the hydrofluoric acid liberated
in the reaction may bring a part of the silica into reaction.
Boron fluoride should be collected over mercury, as water acts on
it, as we shall see afterwards.

[14 bis] It appears to me that from this point of view it is possible
to understand the apparently contradictory results of different
investigators, especially those of Gay-Lussac (and Thénard), Davy,
Berzelius, and Bazaroff. In the form in which the reaction of
BF_{3} on water is given here, it is evident that the act of
solution in water is accompanied by complex but direct chemical
transformations, and I think that this example should prove the
justness of those observations upon the nature of solutions which
are given in Chapter I.

[15] They are called fluoborates. They may be prepared directly from
fluorides and borates. Such compounds of halogens with oxygen
salts are known in nature (for instance, apatite and boracite),
and may be artificially prepared. The composition of the
fluoborates--for example, K_{4}BF_{3}O_{2}--may be expressed as
that of a double salt, BO(OK),3KF. If an excess of water
decomposes them (Bazaroff), this does not prove that they do not
exist as such, for many double salts are decomposed by water.

[16] Fluoboric acid contains boron fluoride and water, hydrofluoboric
acid, boron fluoride, and hydrofluoric acid. It is evident that on
the one side the competition between water and hydrofluoric acid,
and, on the other hand, their power to combine, are among the
forces which act here. From the fact that hydroborofluoric acid,
HBF_{4}, can only exist in an aqueous solution, it must be assumed
that it forms a somewhat stable system only in the presence of
3H_{2}O.

Nothing of this kind occurs with boron chloride, because hydrochloric acid does not act on boric acid. However, amorphous boron at 400° burns in chlorine, and at 410° forms _boron chloride_, BCl_{3}. The boron burns in the chlorine, forming a gas which, in a freezing mixture, condenses into a liquid boiling at 17°, and gives up its excess of chlorine, if there be any, to mercury. The specific gravity of this liquid is 1·42 at 6°. Boron chloride may also be directly obtained from boric anhydride by the simultaneous action of charcoal and chlorine at a high temperature: B_{2}O_{3} + 3C + 3Cl_{2} = 2BCl_{3} + 3CO. It is also obtained by the action of phosphoric chloride on boric anhydride in a closed tube at 200° It is completely decomposed by water, like the chloranhydride of an acid, boric acid being formed; hence it fumes in the air: 2BCl_{3} + 6H_{2}O = 2BH_{3}O_{3} + 6HCl. Boron forms with bromine a similar compound, BBr_{3}, specific gravity at 6° = 2·64, boiling at 90°. The vapour densities of the fluoride, chloride, and bromide of boron show that they contain three atoms of the halogen in the molecule--that is, that boron is a trivalent element forming BX_{3}.[16 bis]

[16 bis] Iodide of boron, BI_{3}, was obtained by Moissan (1891), by
heating a mixture of the vapours of HI and BCl_{3} in a tube, or
by the action of iodine vapour (at 750°) or HI upon amorphous
boron. BI_{3} is a solid substance which dissolves in benzol and
CS_{2}, reacts with water, melts at 43°, boils at 210°, has a
density 3·3 at 50°, and partially decomposes in the light. Besson
(1891) obtained BIBr_{2} (boiling at 125°), and BI_{2}Br (boiling
at 180°) by heating (300-400°) a mixture of the vapours of HI and
BBr_{3}, and showed that NH_{3} combines with BBr_{3} and BI_{3}
in various proportions.

As in the first group lithium is followed by sodium, giving a more basic oxide, so in the second group beryllium is followed by magnesium, and so also in the third group there is, besides the lightest element, boron, whose basic character is scarcely defined, _aluminium_, Al = 27, whose oxide, alumina, has somewhat distinct basic properties, which, although not so powerful as in magnesium oxide, are more distinct than in boric anhydride. Among the elements of the third group, aluminium is the most widely distributed in nature; it will be sufficient to mention that it enters into the composition of clay to demonstrate the universal distribution of aluminium in the earth's crust.

Alumina is so named from its being the metal of alums (_alumen_).

_Clay_, which is so widely distributed and familiar to everybody, is the insoluble residue obtained after the action of water containing carbonic acid on many rocks, and especially on the felspars contained in some of them. Felspar is a compound containing potash or soda, alumina, and silica. The primary rocks, like granite, contain many similar compounds (_see_ Chapter XVIII.: Felspars). Felspar is acted on by water containing carbonic acid, all the alkalis (potash and soda), and a portion of the silica passing into the water as substances which are soluble and carried away by it, whilst the alumina and silica left from the felspar remain on the spot where the solution has taken place. This is the original method of the formation of clay in its primary deposits among rocks along whose crevices the atmospheric water has permeated. Such primary deposits often contain a white pure clay, termed _kaolin_ or _porcelain clay_. But such clay is a rarity, because the conditions for its formation are rarely met with. The water, whilst acting chemically on rocks, at the same time destroys them _mechanically_, and carries off the finely divided residues of disintegration with it. Clay is most easily subjected to this mechanical action of water, because it is composed of grains of exceedingly small size and void of any visible crystalline structure, which easily remain suspended in water. The cloudy water of running mountain streams generally contains particles of clay in suspension, owing to the above-described chemical and mechanical action of the water on the minerals contained in the mountain rocks. Together with these minute particles of clay the water carries away the coarser components on which it is not able to act--for example, splinters of rock, grains of mica, quartz, &c. They were originally held together by those minerals which form clay. When the water acts on these binding minerals, a sandy mass is formed which water bears away. The cloudy water in which the particles of clay and sand are held in suspension carries them to, and deposits them at, the estuaries of rivers, lakes, seas, and oceans. The coarser particles are first deposited and form sand and similar disintegrated rocky matter, whilst the clay, owing to its finely divided state, is carried on further, and is only deposited in the still parts of the rivers, lakes, &c. Such disintegrations of rocks and separations of clay from sand have been gradually going on during the millions of years of the earth's existence, and are now proceeding, and have been the cause of the formation of the immense deposits of sandstone and clay now forming a part of the earth's strata. Such beds of clay may have been transferred by currents and streams from one locality to another, so that we must distinguish between primary and secondary deposits of clay. In places these beds of clay have, owing to long exposure under water, and perhaps partially owing to the action of heat, undergone compression, and have formed the rocky masses known as clay slates and schists, which sometimes form entire mountains. Roofing slates belong to this class of rocks.

From what has been said above it will be evident that these deposits can never consist of a chemically pure and homogeneous substance, but will contain all kinds of extraneous insoluble finely divided matter, and especially sand--that is, fragments of rock, chiefly quartz (SiO_{2}). It is, however, possible to considerably purify clay from these impurities, owing to the fact that they are the result of mechanical disintegration, whilst the clay has been formed as a residue of the chemical alteration of rocky matter, and therefore its particles are incomparably more minute than the particles of sand and other rock fragments mixed with it. This difference in the size of the grains causes the clay to remain longer in suspension when shaken up in water than the coarser grains of sand. If clay be shaken up in water, and especially if it be previously boiled in it, and if after the first portion has settled the cloudy water be decanted, it will give a deposit of a very much purer clay than the original. This method is employed for purifying kaolin designed for the manufacture of the best kinds of china, earthenware, &c. A similar method is also employed in the investigation of earths for determining the _composition of soils_ chiefly composed of a mixture of sand, clay, limestone, and mould. The limestone is soluble in dilute acids, but neither the clay nor sand passes into solution by this means, and therefore the limestone is easily separated in the investigation of soils. The clay is separated from the sand by a mechanical method similar to that described above, and termed _levigation_.[17]

[17] The process of _levigation_ is based on the difference in the
diameters of the particles of clay and sand. In density these
particles differ but little from each other, and therefore a
stream of water of a certain velocity can only carry away the
particles of a certain diameter, whilst the particles of a larger
diameter cannot be borne away by it. This is due to the resistance
to falling offered by the water. This resistance to substances
moving in it increases with the velocity, and therefore a
substance falling into water will only move with an increasing
velocity until its weight equals the resistance offered by the
water, and then the velocity will be uniform. And as the weight of
the minute particles of clay is small, the maximum velocity
attained by them in falling is also small. A detailed account of
the theory of falling bodies in liquid, and of the experiments
bearing on this subject, may be found in my work, _Concerning the
Resistance of Liquids and Aeronautics_, 1880. The minute particles
of clay remain suspended longer in water, and take longer to fall
to the bottom. Heavy particles, although of small dimensions, fall
more quickly, and are borne away by water with greater difficulty
than the lighter. In this way gold and other heavy ores are washed
free from sand and clay, and the coarser portions and heavier
particles are left behind. A current of water of a certain
velocity cannot carry away with it particles of more than a
definite diameter and density, but by increasing the velocity of
the current a point may be arrived at when it will bear away
larger particles. A description of apparatus for the observation
of phenomena of this kind is given by Schöne in his memoir in the
Transactions of the Moscow Society of Natural Sciences for 1867.
In order to be able accurately to vary the velocity of the current
of water, a cylinder is employed in which the earth to be
experimented on is placed, and water is introduced through the
conical bottom of the cylinder. The rate at which the water rises
in the cylinder will vary according to the quantity of water
flowing per unit of time into the vessel, and consequently
particles of various sizes will be carried away by the water
flowing over the upper edges of the vessel. Schöne showed by
direct experiment that a current of water having a velocity of 0·1
mm. per second will carry away particles having a diameter of not
more than 0·0075 mm., that is, only the most minute; with a
velocity _v_ = 0·2 mm. per second, particles having a diameter _d_
= 0·011 mm. are carried away; with _v_ = 0·3 mm., _d_ = 0·0146
mm.; with _v_ = 0·4 mm., _d_ = 0·017 mm.; with _v_ = 0·5 mm., _d_
= 0·02 mm.; with _v_ = 1 mm., _d_ = 0·03 mm.; with _v_ = 4 mm.,
_d_ = 0·07 mm.; with _v_ = 10 mm., _d_ = 0·137 mm.; with _v_ = 12
mm., _d_ = 0·15 mm.; and therefore if the current does not exceed
one of these velocities, it will only carry away or wash away
particles having a diameter less than that indicated. The sand and
other particles mixed with the clay will then remain in the
vessel. The very minute particles obtained after levigation are
all considered as clay, although not only clay but other rock
residue may also exist in it as very fine particles. However, this
is very seldom the case, and the fine mud separated from all clays
has practically the same composition as the purest kinds of
kaolin.

The relation between the amounts of clay and sand in soils used
for the cultivation of plants is very important, because a soil
rich in clay is denser, heavier, shrinks up under the action of
heat, and does not readily yield to the plough in dry or wet
weather, whilst a soil rich in sand is friable, crumbling, easily
parts with its moisture and dries rapidly, but is comparatively
easily worked. Neither crumbling sand nor pure clay can be
regarded as a good _cultivating soil_. The difference in the
amounts of clay and sand in a soil has also a purely chemical
signification. Sand is easily permeated by the air, because its
particles are not closely packed together. Hence the chemical
change of manures proceeds very easily in sandy soils. But on the
other hand such soils do not retain the nutritious principles
contained in the manure, nor the water necessary for the
nourishment of plants by means of their roots. Solutions of
nutritious substances, containing salts of potassium, phosphoric
acid, &c., when passed through sand only leave a portion
moistening the surface of its particles. The sand has only to be
washed with pure water and all the adhering films of solution are
washed away. It is not so with clay. If the above solutions be
passed through a layer of clay the retention of the nutritive
substances of these solutions will be very marked; this is partly
because of the very large surface which the minute particles of
clay expose. The nutritive elements dissolved in water are
retained by the particles of clay in a peculiar manner--that is,
the absorptive power of clay is very great compared to that of
sand--and this has a great significance in the economy of nature
(Chapter XIII., p. 547). It is evident that for cultivation the
most convenient soils in every respect will be those containing a
definite mixture of clay and sand, and indeed the most fertile
soils have this composition. The study of fertile soils, which is
so important for a knowledge of the natural conditions for the
application of fertilisers, belongs, strictly speaking, to the
province of agriculture. In Russia the first foundation of a
scientific fertilisation has been laid by Dokuchaeff. As an
example only, we will give the composition of four soils; (1) The
black earth of the Simbirsk Government; (2) a clay soil from the
Smolensk Government; (3) a more sandy soil from the Moscow
Government; and (4) a peaty soil from near St. Petersburg. These
analyses were made in the laboratory of the St. Petersburg
University about 1860, in connection with experiments on
fertilisation (conducted by me) by the Imperial Free Economical
Society. 10,000 grams of air-dried soil contain the following
quantities (in grams) of substances capable of dissolving in
acids, and of serving for the nourishment of plants.

(1) (2) (3) (4)

Na_{2}O 11 5 4 4
K_{2}O 58 10 7 5
MgO 92 33 19 7
CaO 134 17 14 11
P_{2}O_{5} 7 1 7 3
N 44 11 13 16
S 13 7 7 6
Fe_{2}O_{3} 341 155 111 46

By chemical and mechanical analysis, the chief component parts per
100 parts of air-dried soil are

Clay 46 29 12 10
Sand 40 67 86 84
Organic matter 3·7 1·7 0·6 4·1
Hygroscopic water 6·3 1·3 0·8 1·9
Weight of a litre in grams 1150 1270 1350 960

The black earth excels the other soils in many respects, but
naturally its stores are also exhausted by cultivation if nothing
be returned to it in the form of fertilisers; and the improvement
of a soil (for instance, by the addition of marl or peat, and by
drainage and watering), and its fertilisation, if carried on in
conformity with its composition and with the properties of the
plants to be cultivated, are capable of rendering not only every
soil fit for cultivation, but also of improving its value, so that
in the course of time whole countries (like Holland) may clearly
improve their agricultural position, whilst under the ordinary
_régime_ of continued exhaustion of the soil, entire regions (as,
for instance, many parts of Central Asia) may be rendered unfit
for any agriculture.

By treating clay with strong sulphuric acid, which dissolves the alumina in it, and then (by means of an alkaline carbonate) dissolving the silica which was combined with the alumina in the clay (but not that occurring in the form of sand, &c., which is hardly dissolved by carbonate of soda solution at all even on boiling), we may form an idea of the proportion between the component parts of a clay; and by igniting it at a high temperature, we may determine the amount of water held in it. In the purer sorts of clay dried at 100° (sp. gr. of pure kaolin is about 2·5) this proportion is about 2SiO_{2} : 2H_{2}O : Al_{2}O_{3}. In this case the conversion of felspar into kaolin is expressed by the equation:--

K_{2}O,Al_{2}O_{3},6SiO_{2} = Al_{2}O_{3},2SiO_{2} + K_{2}O,4SiO_{2};
Felspar Kaolin

the compound K_{2}O,4SiO_{2} passes into solution.

But as a rule clays contain from 45 to 60 p.c. of silica, from 20 to 30 p.c. of alumina, and about 12 p.c. of water; and it cannot be supposed that clays are always homogeneous, because they are an aggregation of residues (of silico-aluminous compounds) which are unacted on by water. Nevertheless, clays always contain a hydrous compound of alumina and silica, which is able to give up the alumina contained by it as a base to strong sulphuric acid, forming aluminium sulphate, which is soluble in water. After this treatment the silica remains, and is soluble in a solution of an alkaline carbonate.[18]

[18] Everyone knows that a mixture of clay and water is endowed with
the property of taking a given form when subjected to a moderate
pressure. This plasticity of clay renders it an invaluable
material for practical purposes. From clay are moulded and
manufactured a variety of objects, beginning with the common brick
and ending with the most delicate china works of art. This
_plasticity of clay_ increases with its purity. When articles made
of clay are dried, the well-known hard mass is obtained; but water
washes it away, and furthermore, the cohesion of its particles is
not sufficiently great for it to resist the impression of blows,
shocks, &c. If such an article be subjected to the action of heat,
its volume first decreases, then it begins to lose water, and it
shrinks still further (in the case of a compact mass approximately
by 1/5 of its linear measurement). On the other hand, a great
coherence of particles is obtained, and thus burnt clay has the
hardness of stone. Pure clay, however, shrinks so considerably
when burnt that the form given to it is destroyed and cracks
easily form; such vessels are also porous, so that they will not
hold water. The addition of sand--that is, silica in fine
particles--or of _chamotte_--that is, already burnt and crushed
clay--renders the mass much more dense and incapable of cracking
in the furnace. Nevertheless, such clay articles (bricks,
earthenware vessels, &c.) are still porous to liquids after being
burnt, because the clay in the furnace is only baked and does not
fuse. In order to obtain articles impervious to water the clay
must either be mixed with substances which form a glassy mass in
the furnace, permeating the clay and filling up its pores, or else
only the surface of the article is covered with such a glassy
fusible substance. In the first case the purest kinds of clay give
what is known as china, in the second case porcelain or 'faïence.'
So, for instance, by covering the surface of clay articles with a
layer of the oxides of lead and tin, the well-known white glaze is
obtained, because the oxides of these metals give a white gloss
when fused with silica and clay. In the preparation of china,
fluor spar and finely ground silica is mixed up into the clay;
these ingredients give a mass which is infusible but softens in
the furnace, so that all the particles of the clay cohere in this
softened mass, which hardens on cooling. A glaze composed of
glassy substances, which only fuse at a high temperature, is also
applied to the surface of china articles.

Clay is the source from which alumina, Al_{2}O_{3}, and the majority of the compounds of aluminium are prepared. Among these compounds the most important are the alums--that is, the double sulphates of potassium (and allied metals) and aluminium, AlK(SO_{4})_{2},12H_{2}O. When clay is treated with sulphuric acid diluted with a certain amount of water, aluminium sulphate, Al_{2}(SO_{4})_{3}, is formed; and if potassium carbonate or sulphate be added to this solution, a double salt or alum is obtained in solution. The alums crystallise easily, and are prepared on a very large manufacturing scale owing to their being employed in the process of dyeing. Alums are soluble in water, and, on the addition of ammonia to their solutions, they give _hydrous alumina_, or _aluminium hydroxide_, as a white gelatinous precipitate, which is insoluble in water but easily soluble in acids, even when dilute, and in aqueous soda or potash. The solubility of alumina in acids indicates the basic character of the oxide, and its solubility in alkalis and its power of forming compounds with them shows the weakness of this basic character. However, the feeblest acids, even carbonic acid, take up the alkali from such a solution, and the alumina then separates out in a precipitate as the hydroxide. It must also be remembered as characteristic of the salt-forming properties of alumina that it does not combine with such feeble acids as carbonic, sulphurous, or hypochlorous, &c.--that is, its compounds with these acids are decomposed by water. It is also important to observe that the hydroxide is not soluble in aqueous ammonia.

_Alumina_, Al_{2}O_{3}--that is, the anhydrous aluminium oxide--is met with in nature, sometimes in a somewhat pure state, having crystallised in transparent crystals, which are often coloured by impurities (chromic, cobaltic, and ferric compounds). Such are the ruby and sapphire, the former red and the latter blue. They have a specific gravity 4·0, are distinguished by their very great hardness, which is second only to that of the diamond, and they represent the purest form of alumina. They are found in Ceylon and other islands of the Indian Archipelago, embedded in a rock matrix.[18 bis] _Corundum_ is the same crystallised anhydrous alumina coloured brown by a trace of oxide of iron. A very much larger portion of this impurity occurs in _emery_, which is found in crystalline masses in Asia Minor and in Massachusetts, and owing to its extreme hardness is employed for polishing stones and metals. In this anhydrous and crystalline state the aluminium oxide is a substance which very powerfully resists the action of reagents, and is insoluble both in solutions of the alkalis and in strong acids. It is only capable of passing into solution after being fused with alkalis.[19] Alumina may be obtained in this form by artificial means if the hydroxide be ignited and then fused in the oxyhydrogen flame.[20] Alumina also occurs in nature in combination with water--as, for instance, in the rather rare minerals _hydrargillite_ (sp. gr. 2·3), Al_{2}O_{3},3H_{2}O = 2Al(HO)_{3}, and _diaspore_, Al_{2}O_3,H_{2}O = 2AlO(HO) (sp. gr. 3·4). A less pure hydrate, mixed with ferric oxide, sometimes occurs in masses (at Baux in the south of France) and is termed _bauxite_; it contains Al_{2}O_{3},2H_{2}O = Al_{2}O(HO)_{4} (sp. gr. 2·6). When bauxite is ignited with sodium carbonate, carbonic anhydride is liberated and the alumina then combines with the sodium oxide, forming a saline aluminate of the oxides of aluminium and sodium. This is taken advantage of in practice for the preparation of pure alumina compounds on a large scale, for bauxite is found in large masses (in the South of France, in Austria, and in Carolina in South America), and the resultant compound of alumina and sodium is soluble in water and does not contain ferric oxide. This solution when subjected to the action of carbonic anhydride gives a precipitate of aluminium hydroxide,[21] which with acids forms aluminium salts. If aqueous ammonia be added to a solution of aluminium sulphate a gelatinous precipitate is formed, which at first remains suspended in the liquid and then on settling forms a gelatinous mass, which itself indicates the _colloidal property of aluminium hydroxide_. The following points are characteristic of this colloidal state: (1) in an anhydrous state such a colloidal substance is insoluble in water, as alumina is; (2) in the hydrated state, it is gelatinous and insoluble in water; and (3) it is also capable of existing in solutions, from which it separates out in a non-crystalline state, forming a substance resembling glue. These different states of colloids were distinguished by Graham, who gave them the following very characteristic names. He called the gelatinous form of the hydrate _hydrogel_, _i.e._ a gelatinous hydrate, and the soluble form of the aqueous compound, _hydrosol_, from the Latin for a soluble hydrate. Alumina readily and frequently assumes these states. The gelatinous hydrate of alumina is its hydrogel. It is, as has been already mentioned, insoluble in water, and, like all similar hydrogels, shows not the faintest sign of crystallisation; it is apt to vary in many of its properties with the amount of water it contains, and loses its water on ignition, leaving a white powder of the anhydrous oxide. The hydrogel of alumina is soluble both in acids and alkalis. It may also be obtained by the evaporation of its solutions in such feebly energetic acids as volatile acetic acid. These properties are very frequently made use of in the arts, and especially in _the processes of dyeing_, because the hydrogel of alumina in precipitating attracts a number of colouring matters from their solutions, the precipitate being thus coloured by the dyes attracted.[22] The preparation of fixed dyes and the employment of aluminous compounds (mordants) in the processes of dyeing are founded on this fact.[23] When precipitated upon the fibres of tissues (calicoes, linens, &c.) the aluminium hydroxide renders them impermeable to water; this may be taken advantage of for the preparation of waterproof tissues.

[18 bis] Frémy (1890) obtained transparent rubies, which crystallised
in rhombohedra, and resembled natural rubies in their hardness,
colour, size, and other properties. He heated together a mixture
of anhydrous alumina containing more or less caustic potash, with
barium fluoride and bichromate of potassium. The latter is added
to give the ruby its colour, and is taken in small quantity (not
more than 4 parts by weight to 100 parts of alumina). The mixture
is put into a clay crucible, and heated (for from 100 hours to 8
days) in a reverberatory furnace at a temperature approaching
1,500°. At the end of the experiment the crucible was found to
contain a crystalline mass, and the walls were covered with
crystals of the ruby of a beautiful rose colour. It was found that
the access of moist air was indispensable for the reaction.
According to Frémy, the formation of the ruby may be here
explained by the formation of fluoride of aluminium which under
the action of the moist air at the high temperature of the furnace
gives the ruby and hydrofluoric acid gas.

[19] The effects of purely mechanical subdivision on the solubility of
alumina are evident from the fact that native anhydrous alumina,
when converted into an exceedingly fine powder by means of
levigation, dissolves in a mixture of strong sulphuric acid and a
small quantity of water, especially when heated in a closed tube
at 200°, or when fused with acid sulphate of potassium (_see_